This invention relates to a photosensitive material comprising a silver halide emulsion layer stabilized against over-development fogging.
When a silver halide photosensitive material is subjected to developing treatment under severe conditions such as development at comparatively high temperatures or rapid development in a short period of time by the use of a highly active developer (a developer at a high temperature and a high pH), there is a danger of the accompanied reduction of silver halide grains containing entirely no latent image nuclei. The fog produced under the said conditions by the undesired reduction of unexposed silver halide grains manifests itself especially intensely at the end of common development processing and is called over-development fog.
The antifoggants known to be effective in reducing the over-development fog include mercury compounds and heterocyclic mercapto compounds. Although these antifoggants generally reduce the fogging during the course of development or in case of over development, yet, when used in an amount sufficient for the reduction of above-said fogging, they have also a disadvantage of appreciably decreasing the sensitivity of silver halide photosensitive materials. A basic disadvantage of the direct incorporation of heterocyclic mercapto compounds or others which are generally active by themselves against the over-development fogging into a silver halide emulsion originates in the fact that they are active to full extent from the instant of incorporation, that is, they remain active throughout the manufacturing step, shelf period, and the development step; as a consequence they exhibit an undesirable desensitizing effect on the photosensitive material during the manufacture and shelf period thereof.
In order to solve the above problem, attempts have heretofore been made in the art to protect the mercapto group of the aforementioned compounds with a hydrolyzable group, thus rendering the compound inactive during the period wherein its activity is undesirable (throughout the entire period prior to the development processing, including the manufacturing step), and to reactivate the compound by alkaline hydrolysis in the development step. Such substituent groups are generally moieties of thioesters or thioethers of the aforementioned mercapto-type antifoggants.
Regarding the thioester-type substituent, those of the carboxylic acids, sulfonic acids, and carbonic acid derivatives are disclosed in many patents such as, for example, German Pat. No. 1,597,503, U.S. Pat. No. 3,260,597 and German Patent Application "Offenlegungsschrift" No. 2,061,972. Although hydrolyzable in an alkaline developer medium, these thioesters have a disadvantage of being gradually hydrolyzed at least partially also in neutral or weakly acid region. As a consequence, although can be added in inactive form into the emulsion, these thioester-type antifoggants tend to cause undesirable desensitization owing to the partial hydrolysis in the step of emulsion making or during storage of the photosensitive material, if the time span is sufficiently extended.
As contrasted, although the antifoggants having thioether-type substituents, such as, for example, those disclosed in U.S. Pat. Nos. 2,981,624 and 3,260,597 and German Pat. No. 1,173,796 are indeed completely stable in a neutral or weakly acid medium, yet they are unable to prevent effectively the over-development fogging from occurring, because in the development step they are either unable to regenerate the original mercapto-antifoggant or able to regenerate but very slowly.
Further, those compounds disclosed in U.S. Pat. No. 3,674,478 which are able to release a quinone-methide or naphthoquinone-methide and a mercapto-antifoggant in the presence of alkali are indeed able to release quickly the antifoggant in the presence of alkali, but are disadvantageous in that in a weakly acid medium they are unstable to some degree, gradually releasing the antifoggant. As a consequence, if the time interval between the preparation and coating of an emulsion (having a pH in the weakly acid region) is sufficiently long, the antifoggant will be entirely released into the emulsion, causing undesirable desensitization.